Biological classification

• Since the dawn of civilisation, there have been many attempts to classify living organisms
• Aristotle was the earliest to attempt a more scientific basis for classification
• Aristotle's classification

  • Used simple morphological characters to classify plants into trees, shrubs and herbs
  • Divided animals into two groups, those which had red blood and those that did not
• In Linnaeus' time a Two Kingdom system of classification with Plantae and Animalia kingdoms was developed
• The two kingdom classification was found inadequate as a large number of organisms did not fall into either category
• Besides gross morphology, a need was also felt for including other characteristics like cell structure, nature of wall, mode of nutrition, habitat, methods of reproduction, evolutionary relationships, etc.
• Classification systems for the living organisms have undergone several changes over time
• Though plant and animal kingdoms have been a constant under all different systems, the understanding of what groups/organisms be included under these kingdoms have been changing
• The number and nature of other kingdoms have also been understood differently by different scientists over time
• Five Kingdom Classification

  Kingdoms defined by R.H. Whittaker in 1969: Monera, Protista, Fungi, Plantae and Animalia
• Criteria used by Whittaker for classification

  • Cell structure
  • Body organisation
  • Mode of nutrition
  • Reproduction
  • Phylogenetic relationships
• The three-domain system has also been proposed that divides the Kingdom Monera into two domains, leaving the remaining eukaryotic kingdoms in the third domain and thereby a six kingdom classification
• Earlier classification systems included bacteria, blue green algae, fungi, mosses, ferns, gymnosperms and the angiosperms under 'Plants'
• The character that unified the 'Plant' kingdom was that all the organisms included had a cell wall in their cells
• This placed together groups which widely differed in other characteristics, like the prokaryotic bacteria and the blue green algae with the eukaryotic and multicellular organisms
• The classification did not differentiate between the heterotrophic group - fungi, and the autotrophic green plants, though they also showed a characteristic difference in their wall composition
• When such characteristics were considered, the fungi were placed in a separate kingdom - Kingdom Fungi
• All prokaryotic organisms were grouped together under Kingdom Monera and the unicellular eukaryotic organisms were placed in Kingdom Protista
• Kingdom Protista has brought together organisms which, in earlier classifications, were placed in different kingdoms
• This happened because the criteria for classification changed
• Over time, an attempt has been made to evolve a classification system which reflects not only the morphological, physiological and reproductive similarities, but is also phylogenetic, i.e., is based on evolutionary relationships
• Shapes of bacteria

  • Cocci (spherical)
  • Bacilli (rod-shaped)
  • Vibrio (comma-shaped)
  • Spirilla (spiral)
• Bacteria

  • They are the most abundant micro-organisms
  • They occur almost everywhere, including extreme habitats
  • Many live in or on other organisms as parasites
• Autotrophic bacteria


  They synthesise their own food from inorganic substrates, either photosynthetically or chemosynthetically
• Heterotrophic bacteria


  They depend on other organisms or on dead organic matter for food
• Archaebacteria


  • They live in harsh habitats like extreme salty areas, hot springs and marshy areas
  • They have a different cell wall structure which helps them survive in extreme conditions
  • Methanogens in the gut of ruminant animals produce methane (biogas)
• Eubacteria

  • They have a rigid cell wall and, if motile, a flagellum
  • Cyanobacteria are photosynthetic autotrophs
  • Chemosynthetic autotrophic bacteria oxidise inorganic substances and use the released energy for ATP production
  • Heterotrophic bacteria are important decomposers, some are pathogens causing diseases
• Bacterial reproduction

  1. Fission

  2. Spore formation

  3. Primitive type of DNA transfer (sexual reproduction)
• Mycoplasma


  Organisms that completely lack a cell wall, the smallest living cells known, can survive without oxygen, many are pathogenic
• Members of Protista are primarily aquatic
• Protista


  • They are single-celled eukaryotes
  • Their cell body contains a well defined nucleus and other membrane-bound organelles
  • Some have flagella or cilia
  • They reproduce asexually and sexually by a process involving cell fusion and zygote formation
• Groups included under Protista

  • Chrysophytes
  • Dinoflagellates
  • Euglenoids
  • Slime moulds
  • Protozoans
• Chrysophytes

  • They include diatoms and golden algae
  • They are microscopic and float passively in water currents (plankton)
  • Most are photosynthetic
  • Diatoms have cell walls embedded with silica, forming two overlapping shells
• Protista


  A kingdom that includes Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds and Protozoans
• Protista


  • Primarily aquatic
  • Eukaryotic with well-defined nucleus and membrane-bound organelles
  • Some have flagella or cilia
  • Reproduce asexually and sexually
• Chrysophytes


  A group that includes diatoms and golden algae
• Chrysophytes

  • Diatoms
  • Golden algae (desmids)
• Chrysophytes

  • Found in fresh water and marine environments
  • Microscopic and float passively in water currents (plankton)
  • Mostly photosynthetic
  • Diatoms have silica cell walls that accumulate as 'diatomaceous earth'
  • Diatoms are chief producers in oceans
• Dinoflagellates


  Mostly marine and photosynthetic organisms
• Dinoflagellates


  • Cell wall has stiff cellulose plates
  • Have two flagella, one longitudinal and one transverse
  • Red dinoflagellates (e.g. Gonyaulax) can cause 'red tides' and release toxins